1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Background of the Related Art
For example, an insulated gate bipolar transistor (IGBT) or a diode with a breakdown voltage of 400 V, 600 V, 1200 V, 1700 V, 3300 V, or more has been known as a power semiconductor device. The power semiconductor device is used in a power conversion device, such as a converter or an inverter.
The following method has been known as a method for manufacturing the power semiconductor device. First, a front surface element structure is formed on a front surface of a semiconductor substrate. Then, a rear surface of the semiconductor substrate is removed by, for example, grinding to reduce the thickness of the semiconductor substrate. Then, impurity ions are implanted into the ground rear surface of the semiconductor substrate. Then, a heat treatment is performed to activate the impurities implanted into the rear surface of the semiconductor substrate to form a rear surface element structure. In addition, as this type of method, various methods have been proposed which radiate protons to the semiconductor substrate and perform a heat treatment to form an n+ layer with high concentration in the semiconductor substrate, using a hydrogen induced donor (or simply a hydrogen donor) obtained by changing a composite defect of radiated (implanted) hydrogen atom and a peripheral vacancy into a donor.
A method for manufacturing a trench gate IGBT, which is an example of the semiconductor device, using the activation of the proton by the heat treatment will be described. FIGS. 26 to 31 are cross-sectional views illustrating the semiconductor device according to the related art which is being manufactured. FIG. 32 is a cross-sectional view illustrating the semiconductor device after a process which follows the process illustrated in FIG. 31. First, as illustrated in FIG. 26, an n− semiconductor substrate which will be an n− drift layer 101 is prepared. Then, as illustrated in FIG. 27, a trench-gate-type MOS gate (metal-oxide-semiconductor insulated gate) structure including a p base region 102, an n+ emitter region 103, a trench 104, a gate oxide film 105, and a gate electrode 106 are formed on the front surface of the semiconductor substrate. Reference numeral 108 is an interlayer insulating film.
Then, as illustrated in FIG. 28, an emitter electrode 107 is formed so as to come into contact with the p base region 102 and the n+ emitter region 103. Then, the rear surface of the semiconductor substrate is removed by, for example, grinding to reduce the thickness of the semiconductor substrate. Then, a proton (H+) 121 is radiated to the ground rear surface of the semiconductor substrate. In FIG. 28, an “x” mark in the vicinity of the rear surface of the semiconductor substrate indicates the radiated proton (which holds for FIGS. 7, 11, 18, 22, 48, 50, and 52). Then, as illustrated in FIG. 29, the proton 121 radiated to the semiconductor substrate is activated by annealing to form an n field stop (FS) layer 110 in the n− drift layer 101 in the vicinity of the rear surface.
Then, as illustrated in FIG. 30, boron ions (B+) 122 are implanted into a region that is shallower than the n field stop layer 110 from the rear surface of the semiconductor substrate. In FIG. 30, a dotted line in the vicinity of the rear surface of the semiconductor substrate indicates the implanted impurity ion (which holds for FIGS. 5, 9, 16, 20, 37, and 53). Then, as illustrated in FIG. 31, the boron ions 122 implanted into the semiconductor substrate are activated by annealing to form a p+ collector layer 109 in a surface layer of the rear surface of the semiconductor substrate. Then, as illustrated in FIG. 32, a collector electrode 111 is formed so as to come into contact with the p+ collector layer 109. In this way, the trench gate IGBT is completed.
A method for manufacturing a diode, which is an example of the semiconductor device, using the activation of the proton by the heat treatment will be described. FIGS. 33 to 38 are cross-sectional views illustrating another example of the semiconductor device which is being manufactured. FIG. 39 is a cross-sectional view illustrating the semiconductor device after a process which follows FIG. 38. First, as illustrated in FIG. 33, an n− semiconductor substrate 131 is prepared. Then, as illustrated in FIG. 34, a p anode region 132 is formed in a front surface of the semiconductor substrate 131. Reference numeral 134 is an interlayer insulating film.
Then, as illustrated in FIG. 35, an anode electrode 133 is formed on the front surface of the semiconductor substrate 131 so as to come into contact with the p anode region 132. Then, as illustrated in FIGS. 35 and 36, similarly to the IGBT manufacturing method, a rear surface of the semiconductor substrate 131 is ground and proton irradiation and annealing are performed to an n field stop layer 136 in the semiconductor substrate 131 in the vicinity of the rear surface. Then, as illustrated in FIG. 37, phosphorus ions (P+) 123 are implanted into a region that is shallower than the n field stop layer 136 from the rear surface of the semiconductor substrate 131.
Then, as illustrated in FIG. 38, the phosphorus ions 123 implanted into the semiconductor substrate 131 are activated by annealing to form an n+ cathode layer 135 in a surface layer of the rear surface of the semiconductor substrate 131. Then, as illustrated in FIG. 39, a cathode electrode 137 is formed so as to come into contact with the n+ cathode layer 135. In this way, the diode is completed. That is, in both the IGBT manufacturing method and the diode manufacturing method, after proton irradiation and annealing for forming the n field stop layer are performed, ion implantation and annealing for forming the rear surface element structure on the rear surface of the semiconductor substrate are performed.
As the semiconductor device manufacturing method, a method has been proposed which radiates protons to a semiconductor substrate, performs a heat treatment to form a high-concentration n+ buffer layer (field stop layer), and implants p-type impurities, such as phosphorus (P), to form a p− collector region, for example, see U.S. Pat. No. 7,956,419 (Patent Document 1). In Patent Document 1, the dose of the p-type impurities, such as phosphorus or arsenic (As), is intentionally reduced and annealing is performed at an annealing temperature lower than the annealing temperature which is most suitable to form a high-concentration collector region to form the p− collector region.
US 2006/0081923 A (Patent Document 2) discloses the structure of an IGBT in which protons are radiated to a rear surface of a semiconductor substrate a plurality of times to form a plurality of field stop layers including hydrogen donors and, particularly, the depth of the field stop layer, which is arranged at the deepest position from the rear surface of the substrate, from the rear surface of the substrate is 15 μm. In this structure, a plurality of field stop layers are formed in the region extending from the rear surface of the substrate to a depth of 15 μm and, particularly, the field stop layer close to the rear surface of the substrate has a high impurity concentration of about 1×1016/cm3. Therefore, the field stop layer can reliably prevent the spreading of a depletion layer and it is possible to prevent the depletion layer from reaching the p collector layer.
US 2006/0035436 A (Patent Document 3) discloses the following method as the IGBT manufacturing method. A MOS gate structure is formed on the front surface side of a semiconductor substrate and the rear surface of the substrate is removed by, for example, grinding to reduce the thickness of the semiconductor substrate. Then, protons are radiated to the ground surface (rear surface) of the semiconductor substrate and an annealing process is performed to form a field stop layer. Then, boron ions are implanted into the rear surface of the semiconductor substrate and laser annealing is performed to form a p-type collector layer.
JP 2009-176892 A (Patent Document 4) discloses the following method as the IGBT manufacturing method. A MOS gate structure is formed on the front surface side of a semiconductor substrate and a rear surface of the semiconductor substrate is removed by, for example, grinding to reduce the thickness of the semiconductor substrate. Then, protons are radiated to the ground surface (rear surface) of the semiconductor substrate. Then, an annealing process using laser beams with two wavelengths, which are emitted from a pulsed laser and a semiconductor continuous-wave laser, is performed for the proton-radiated surface (the rear surface of the substrate) to form a field stop layer at a depth of about 15 μm from the proton-radiated surface.
However, in Patent Document 1, it is difficult to increase the annealing temperature of all annealing processes, which are performed after the proton irradiation, to 500° C. or more in order to set the impurity concentration of the high-concentration n+ buffer layer formed by the proton irradiation and the annealing to a desired value. The reason is that, when the annealing temperature is equal to or higher than 500° C., lattice defects formed by the proton irradiation are reduced and the concentration of the hydrogen induced donors is reduced. In order to ensure the sufficient concentration of the hydrogen induced donors, the annealing temperature may be equal to or higher than 300° C. and equal to or lower than 500° C., particularly, equal to or lower than 400° C. In this case, it is possible to obtain 10% or more donor change rate with respect to the dose of the radiated protons.
When the annealing temperature of all annealing processes which are performed after the proton irradiation is equal to or lower than 400° C., particularly, equal to or lower than 350° C., the amount of heat for activating the impurities is insufficient in the annealing process after the impurity implantation for forming a contact is performed with a high dose required to form the ohmic contact between the semiconductor layer and the rear surface electrode and a high-concentration contact portion required for the ohmic contact with the electrode is not obtained. Therefore, there is a concern that deterioration of electrical characteristics, such as a reduction in contact resistance or a reduction in the on-voltage (Von) of the IGBT or the diode, will occur.
In Patent Document 2, the field stop layer including the hydrogen donors is formed at a depth of 15 μm from the proton-radiated surface (the rear surface of the substrate). However, the field stop layer needs to be formed at a position that is deeper than 15 μm from the proton-radiated surface, in order to improve the electrical characteristics of the IGBT or the diode. However, the inventors found that, when the average range of proton irradiation (the distance of the position where the concentration of the radiated ions was the highest from the irradiation surface) was set to 15 μm or more in order to form the field stop layer at the position that was deeper than 15 μm from the rear surface of the semiconductor substrate, a proton passage region extending from the rear surface of the semiconductor substrate to a depth of 15 μm was a region in which carrier concentration measured by a spreading resistance (SR) profiling method was significantly lower than the doping concentration of the semiconductor substrate, that is, a disorder region.
The defects introduced by the proton irradiation mainly remain in the proton passage region from the proton-radiated surface to the average range or in the vicinity of the proton-radiated surface, in addition to the position corresponding to the average range of the protons. The residual defect is in a state close to an amorphous state due to the large deviation of an atom (in this case, a silicon atom) from a lattice position and the strong disorder of a crystal lattice. Therefore, the residual defect becomes the scattering center of a carrier, such as an electron and a hole. In this case, the characteristics of the element deteriorate as follows: carrier mobility is reduced; and as electrical resistance increases, the amount of leakage current increases since the residual defect is the generation center of a carrier. As such, the defect which remains in the proton passage region extending from the proton-radiated surface to the average range due to proton irradiation, causes a reduction in the carrier mobility or a reduction in leakage current, and is strongly disordered from the crystal state is particularly referred to as a disorder.
FIG. 40 is a characteristic diagram illustrating the relationship between carrier concentration and the average range of proton irradiation according to the related art. FIG. 40 illustrates the carrier concentration of a silicon substrate which is measured by the SR method after protons are radiated to the silicon substrate and a heat treatment is performed at a temperature of 350° C. FIG. 40(a) illustrates a case in which the average range of proton irradiation is 50 μm, FIG. 40(b) illustrates a case in which the average range of proton irradiation is 20 μm, and FIG. 40(c) illustrates a case in which the average range of proton irradiation is 15 μm. In FIGS. 40(a) to 40(c), the horizontal axis is the distance (depth) from a proton incident surface (the rear surface of the substrate). When the average range of proton irradiation is 15 μm as illustrated in FIG. 40(c), the carrier concentration of the proton passage region is not particularly reduced. In contrast, when the average range of proton irradiation is 20 μm as illustrated in FIG. 40(b), the carrier concentration of the proton passage region is reduced and is lower than the substrate concentration. That is, the disorder remains in the region in which the carrier concentration is lower than the substrate concentration. In addition, when the average range of proton irradiation is 50 μm as illustrated in FIG. 40(a), the carrier concentration of the proton passage region is significantly reduced and a large amount of disorder remains. As such, when there is a disorder region in the semiconductor substrate, the amount of leakage current or conduction loss increases, as described above. Therefore, it is necessary to remove the disorder.
In the method disclosed in Patent Document 3, some of the disorder can be reduced by adjusting the conditions, such as the temperature or time of the annealing process after proton irradiation. However, when the layer into which boron ions are implanted from the rear surface of the substrate is activated in order to sufficiently ensure the concentration of the hydrogen induced donors, the temperature of the annealing process in an electric furnace needs to be lower than the annealing temperature after proton irradiation. Therefore, the implanted boron ions are not activated and it is difficult to obtain the ohmic contact with the rear surface electrode in the subsequent process. When the laser annealing is performed after the boron implantation as described in Patent Document 3, the temperature at a depth of about 10 μm from the laser irradiation surface (the rear surface of the substrate) is equal to or higher than about 800° C. for about 10 μs after the laser beam is radiated. The boron ions are sufficiently activated to form the ohmic contact by the thermal budget (thermal energy). However, in particular, when a plurality of field stop layers are formed in the range extending from the rear surface of the substrate to a depth of about 10 μm, the temperature of the region extending from the rear surface of the substrate to a depth of about 10 μm is increased to 800° C. or more by the heat of the laser beam in a short time. A large number of hydrogen donors vanish and it is difficult to obtain sufficient hydrogen donor concentration. As a result, it is difficult to ensure the donor concentration of the field stop layer at a depth of about 10 μm from the rear surface of the substrate.
In the method disclosed in Patent Document 4, the boron layer in the rear surface of the substrate can be activated. However, when the field stop layer is formed at the position that is deeper than 15 μm from the rear surface of the substrate using the hydrogen donors, it is difficult to sufficiently reduce the disorder. When a plurality of field stop layers are formed at a position that is shallower than 15 μm from the rear surface of the substrate using the hydrogen donors, it is difficult to ensure sufficient donor concentration as in Patent Document 3.
In order to solve the above-mentioned problems of the related art, an object of the invention is to provide a method for manufacturing a semiconductor device which can prevent deterioration of electrical characteristics.